Formation of porous rubber products



4 Aug. 3, 1943. w. E. STEWART 2,325,637

FORMATION OF POROUS RUBBER PRODUCTS Filed June 23, 1938 F/GJ 7 H 6 2 F/GZ /l/l/ /l/ INVZWRDR MA; a? ?IM Pateted Aug. 3, 1943 UNITED STATES PATENT OFFICE FORMATION OF POEOUS BUBBEB. PBODUCTS wum 1:. stema-t; Oyalota Flls, Ohio. aslgnor ot one-hai! to Theodore A.- 'l'e Grotenluis, Olmstcad falls. Ohio Application June 23, 1938, Serial No. 215,&19

Claims. c. 260-723) This invention is a continuation in part of my prior application serial No. 104.591, filed October 8, 1936, and relates to a process for manufacturing porous rubber products from compounded latex mixtures. It particularly relates to a process wherein electrical current is used.

Porous rubber products. such as sponge tub ber cushions, etc., have heretofore been made by processes which comprise forming a froth ot a compounded latex nixture, mixing a gelling agent with the froth, pouring the froth into a closed mold, allowing it to gel, andcuring the product by immersing the mold in hot water or steam. These processes are not entirely satisfactory because it is dimcult to cure the center of thick articles, and the curing temperature can not be raised above the boiling point of water without destroying the froth; hence, a long curing time is necessary. They possess further disadvantages because expensive non-cor- I have found that porous rubber products of by passing an electrical current through a froth of compounded latex or artificial rubber dispersions. I have also found that, when the products are cured electrically, the curing temperature may be raised considerably without destroying the porosity of the products. I have further found that while a gelling agent may be added to the froth .it is unnecessary in my process.

The mechanism of gellation without a gelling agent is not definitely known but probably is due to the vaporization of stabilizing agents from the films of the bubbles. Since the gaseous material of the froth is effectively a non-conductor, the exceedingly thin films formed from thewalls of the bubbles must carry all the current. If the current is of considerable magnitude the film of latex is heated much more rapidly than the interior of the bubbles and the' ammonia or other stabilizing agent is vaporizd from the films and gellation takes place whether or not a gelling agent had been added to the froth. Variations in surface temperatures, which cause the collapse of ungelled bubbles when heat is applied as heat, do not occur because the electrical current is applied almost uniformly over bubble surfaces.

The accompanying drawing illustrates a mold which is suitable for forming sponge rubber products by my process.

through a mold for containing p a compounded latex mixture and showing the electrical connections for passing an electric current through the compounded latex mixture in order to gel and cure it. e A v Figure 2 is avertical section through the mold on the line II-II of Figure 1.

Referring more particularly to the drawing,

Figures 1 and 2 illustrate a mold for forming a block' of sponge rubber from compounded latex mixtures. The mold comprises the metallic electrodes 2 and 3 'separated by the side members 4 which may be constructed of Wood or other insulating material suitably treated to prevent absorption of latex. The electrodes 2 and 3, which resp'ectively form the top and bottom of the mold are removably fastened to the side pieces 4 by means of screws or by other convenient meansin such a manner as to provide a leak-proof mold. An electromotive force from a suitable -generator 6 is impressed between the conductors 'I and 8 which make electrlcal contact with the electrodes 2 and 3.

` In the preparation of the porous products by my process an emulsion or dispersion of rubber, 'ba1ata, gutta percha, or a mixture of these.or similar resins with suitable compounding and frothing ingredients is ormed into a froth or' is then replaced and fastened to the side pieces 4 in such a manner as to prevent the escape of appreciable vapor from the mold. An altemating electromotive force impressed between the electrodes 2 and 3, causes a current to flow through the froth 5 which is first gelled or coagulated and by further passage of current it may be completely cured. i

` The voltageapplied between the electrodes 2 and 3 may be varied widely without producing unsatisfactory coagulation or cui-ing. It is preferable to apply a sufllcient voltage to raise the temperature of the froth rapidly to the curing temperature desired and to then cut the voltage just sufliciently to maintain that temperature. Thus, with a mold containing electrodes spaced 4 inches apart, a voltage of 110 volts may be applied. The initial current varies according to the density of the froth and may be in excess of 30 amperes per square foot of electrode area,

Figure 1 is a vertical longitudinal section but this is reduced considerably by increased resistance due to a slight polarization of the electrodes. When the desired' temperature is reached, the voltage is 'adjusted to a value sufficient to maintain this temperature. The material is held at the vulcanization temperature for a sufflcient time to wholly cure the product or to cure it to a degree such that it can` be removed from the mold without permanently deforming it.

These ingredients are mixed together and the mixture milled for 48 hours in a ball mill. The resulting dispersion is used as a oompounding reagent for latex batches, and it is henceforth, designated as solution A.

Example 2 Cubic centimeters Latex (60% solution) 1,200 Solution A 120 Zinc oxide (20% dispersion) 6 Potassium sulflde (10% solution) 6 Gelling agent (10% ammonium sulfate solution) 3 The latex and solution A, prepared in Example 1, are whipped or beaten into a froth which occupies several times the Volume of the original liquid. The aqueous dispersion of zine oxide and potassium sulfide. which contains the gelling agent, is then thoroughly mixed into the froth; whereupon, the mixture is poured into a mold 1 inch thi-ok with electrodes spaced 1 inch apart. A 50 volt, 60 cycle, alternating E. M. F. is impressed between the electrodes, which are constructed of tin and have an area of 325 square inches. The initial current is 45 amperes. After 10 minutes, the current has decreased to 23 amperes and after 20 minutes the current is but 11 amperes and the material is well cured.

Although the preferred process is to wholly cure the porous rubber products in the mold by means of the electrical current, they may be partially cured in the mold and nish-cured by floating them in hot water. The latter method removes the soluble rancid-smelling material while the article is being flnish-cured and the molds are not utilized for as long a period of time.

The gelling'agent in the above examples is desirable but may be omitted; the latex mixture in the above examples may be replaced by any latex mixture which is suitable for making sponge rubber articles by other processes. If desired, the frothed material containing suicient gelling agent may be allowed to gel in the mold before the electrical current is passed through the mixture. The current is then used solely to cure the products. S

In my process the rubber is gelled in position; it is not plated out of the liquid on the electrodes as is the case with the so-called electro-deposition processes. Direct current can be used in my process with a degree of success, but alternating current is much more satisfactory. When direct current is used a greater amount of polarization takes place and a very thin film of rubber is plated on the electrodes when the initial voltage is applied and before gellation takes place.

Aqueous dispersions of coagulated rubber, reclaim rubber, or vulcanized rubber may be employed as alternatives or as admixtures to the natural rubber, balata, or gutta percha dispersions. According y, the term, rubber-like material, as used in the appended claims includes these materials. Curing temperatures well above the boiling point of water may be used in my process without adversely affecting the porosity of the finished product.

In the appended claims, the term "commercially transformable altemating current" is intended to include only alternating currents which may be passed through iron core transformers, and modified in value, without having high loss. Radio frequency Currents, which' cannot be transformed in iron core transformers and which are accordingly very ineflicient for commercial processes, are not, therefore, included within the term.

I claim:

1. In a process for producing porous rubber articles wherein a. liquid containing a rubber latex and suitable oompounding ingredients is `formed into a froth and the froth gelled and Vulcanized in a mold, the step which comprises passing an electric current through the froth in sufficient quantity to gel and cure it in cellular form having the shape of the mold.

2. In a process for producing porous rubber articles wherein a liquid containing a rubber latex is formed into a froth and the froth gelled and cured, the step which comprises passing an electric current through the froth in suflicient quantity and for a suflicient time to vulcanize the froth in cellular form.

3. In a process for producing porous rubber articles wherein a liquid containing rubber latex and suitable oompounding ingredients is formed into a froth and the froth gelled and cured, the step which comprises passing an electrical current through the froth in suflicient quantity to raise the temperature of the froth to a curing temperature and for a suflicient time to cure the froth to such an extent that it can be removed from a mold wihout permanently destroying the cellular structure.

4. A process for producing sponge rubber products from rubber-like material which contains suitable oompounding ingredients, which comprises forming an aqueous dispersion of a rubber into a froth, and passing anelectric current through the froth in suflicient quantity to gel and cure it.

5. A process for producing sponge rubber products which comprises forming a froth of 'an aqueous dispersion of a rubber which contains suitable oompounding ingredients and a gelling agent, adding the froth to a mold, allowing it to gel, and passing an alternating electric current through the froth in suflicient quantity and for sufiicient time to vulcanize the froth permanently in the shape and contour of the mold.

6. A process for producing sponge rubber products which comprises forming a froth of a liquid containing rubber latex and suitable compounding ingredients, adding the froth to a mold and passing an alternating current through the froth in suflicient quantity to gel and cure it.

'1. A process for producing sponge rubber products which comprises forming a liquid containing' rubber latex and suitable compounding ingredients into a froth, adding the froth to a mold, and gelling the froth in the absence of a gelling agent by passing an alternating electric current thru the froth.

8. A process for producing sponge rubber products from rubberlike material which contains suitable compoundingingredients, which comprises forming an aqueous dispersion of a rubber into a froth, and passing a commercially transformable alternating current through the froth insuillcient quantity to gel and cure it in cellular form.

9. A process for producing sponge rubber products which comprises forming a froth of an aqueous dispersion of a rubber which contains suitable compounding ingredents and a gelling agent, adding the froth to a mold, allowing it to gel, and passing a commercially transformable alternating electric current through the froth in sufilcient quantity and for sufilcient time to vulcanize the froth permanently in the shape and contour of the mold.

10. A process for producing sponge rubber` products which comprises forming a iroth of a ducting electrodes, and then passing an alternating electric current between the conductmg electrodes and through the rrethin a sumclent quantityto vulcanize it in cellular form.

12. A process for producing sponge rubber products which comprises forming an aqueous dispersion of a rubber into a foam, disposing the foam between and in 'contact with suitable electrodes, and passing an alternating electric current through the foam between the electrodes in sufilcient quantity and at sufllcient rate to prevent the form from revertlng to the liquid state and causing it to permanently retain the shape 'lt occupied between the electrodes.

13. A process for producing sponge rubber products which comprisesforming an aqueous dispersion of rubber into a foam, disposing the foam between and in contact with suitable electrodes, and passing an electric current through the foam between the electrodes at a sufllciently high rate to cause curing of the foam in cellular form in situ, the degree of said curing being such that the material may be removed from between the electrodes without destroying its cellular WILLIAM E. STEWART. 

